An electronic circuit is chemically and physically integrated into a substrate such as a silicon wafer by patterning regions in the substrate, and by patterning layers on the substrate. These regions and layers can be conductive, for conductor and resistor fabrication, or insulative, for insulator and capacitor fabrication. They can also be of differing conductivity types, which is essential for transistor and diode fabrication. Degrees of resistance, capacitance, and conductivity are controllable, as are the physical dimensions and locations of the patterned regions and layers, making circuit integration possible. Fabrication can be quite complex and time consuming, and therefore expensive. It is thus a continuing quest of those in the semiconductor fabrication business to reduce fabrication times and costs of such devices in order to increase profits. Any simplified processing step or combination of processes at a single step becomes a competitive advantage.
One processing step is referred to as "in situ etch". "in situ etch" includes patterning photoresist on top of an oxide (SiO.sub.2) and silicon (usually deposited polysilicon, or "poly") sandwich, followed by an etch process. Both the oxide and the subjacent poly are etched inside a single etch chamber, followed by resist strip. The present invention is directed to controlling etch patterns during in situ etch.
In this disclosure, "sccm" denotes standard cubic centimeters per minute, and "gap" refers to the distance between plasma electrodes, one of which supports the wafer. After the oxide etch step, which takes under a minute, the structure appears as shown in FIG. 2.
A situation where a process simplification is desirable is in the anisotropic etch of a layer of oxide on a layer of polysilicon (poly). Oxide is an insulator with dielectric properties, and is often layered with other dielectrics such as nitride. Poly is resistive in nature, but is made less resistive when doped with an element having less or more than four valence electrons, or when layered with conductive silicide.
A combination of oxide and poly presents a difficult etching task, particularly with an additional mask layer of photoresist ("resist"), which must be the case if patterning is desired. The difficulty is due to the distinct differences in the way oxide and poly are etched, particularly with resist still present on top of the structure.
While oxide formation is useful in selective etching of poly, oxidizing materials are generally not used when etching silicon past a photomask because photoresist masks do not withstand oxygen environments very well.
Both oxide and poly can be etched using a parallel plate plasma reactor. However, an oxide is typically etched in fluorine deficient fluorocarbon based plasmas, whereas poly can be etched in fluorine or chlorine based discharges. Reactor cathode materials may also differ. Using conventional methods, the two steps are incompatible.
Oxide etch in general is fairly well understood, given a universal need for a vertical profile. This vertical profile is realized primarily by ion induced reaction with the oxide, coupled with normal incidence of the ions onto the oxide surface. The amount and energy of these ions are primarily controlled by the reactor's rf power and gap. Generally, a fluorocarbon-based gas mixture is introduced at a low pressure into the etch chamber. The exact gas composition is chosen, and an erodible cathode is used to scavenge excessive fluorine radicals so that the fluorine-to-carbon ratio is near, but not beyond, the so-called polymerization point. Under these conditions, when a plasma is ignited, the fluorocarbons are dissociated and release fluorine radicals and CF.sub.x species. Although fluorine radicals etch oxide, they do so very slowly: the primary etchant for oxide is considered to be the CF.sub.x species. Some of these species diffuse to the oxide surface where, with the assistance of ion bombardment, they react with the oxide and release volatile byproducts SiF.sub.4, CO, and CO.sub.2. In addition, some of the CF.sub.x species react with each other to form fluorocarbon polymers. Polymer that forms on horizontal surfaces is removed by vertical ion bombardment. Polymer that forms on vertical sidewalls is not significantly degraded by the bombardment, and actually serves a useful purpose by protecting the sidewalls from attack by the etchant species. This sidewall protection enables the achievement of vertical profiles, adjustable by varying the fluorine-to-carbon ratio. As the cathode is eroded, the quantity of available fluorine radicals is reduced. Therefore, a polymer-producing gas such as CHF.sub.3 is balanced against a fluorine producing gas such as CF.sub.4 to provide proper selectivity, with assistance to sidewall protection.
Two methods are presently used to etch an oxide/silicide/poly sandwich structure. Both methods use separate reactors: one for oxide etch, and one for polycide etch. The first method involves etching the oxide in an oxide etch reactor, using an erodible cathode. After oxide etch, the resist is removed from the wafer. Silicide and poly are then etched in a poly etch reactor, using an inert cathode. Both etches are anisotropic.
The second method uses the same principles as the first, except that there are two reactors in one machine. The two reactors are configured as separate oxide and polycide reactors having a common vacuum transfer area, so that a wafer can be transferred in a vacuum from the oxide reactor to the polycide reactor, thus minimizing additional handling. The resist is generally not removed prior to polycide etch in this method. This is sometimes referred to as "in situ" since the wafers never leave the vacuum of one machine. However, they are etched in two different etch chambers and are not truly in situ in the sense of this disclosure.
It would be of great advantage to etch oxide and polycide truly "in situ", that is, in one reactor chamber, with a single cathode. An object of the present invention is to provide a method of anisotropically etching an oxide/silicide/poly sandwich structure in situ. Other objects of the invention are a fast processing time, and improved process yield and cleanliness.
In the in situ etch process, it would be desireable to allow anisotropic etching of the silicon after the oxide is patterned and after resist is stripped. It is further desireable to improve cleanliness and repeatability in order to simplify the process and provide tighter controls over dimensions.